Thimerosal is a mercury-based preservative once common in vaccines. It's now found only in small amounts in some flu vaccines. However, thimerosal was once used in most vaccines given to children.

Although the amounts were tiny, they added up. Because symptoms of autism often appear suddenly around age 1 -- when kids have received a number of vaccinations -- many parents became convinced that thimerosal caused their children's autism. But the Institute of Medicine has twice rejected this idea.

The new CDC study carefully avoided the still-controversial issue of whether thimerosal is linked to autism. A separate CDC study of this issue is under way, with a report expected in a year's time.

"In this study there is nothing you can draw on regarding any relationship to autism," researcher William W. Thompson, PhD, of the CDC's National Immunization Program, said at a news conference.

Instead, the study looked for 42 different neurological and psychological problems in more than 1,000 7- to 10-year-old children exposed to various amounts of thimerosal while still in the womb, from birth to 28 days, or up to age 7 months.

"This was a large, very careful study to determine whether thimerosal exposures in early childhood was associated with bad neuropsychological outcomes," Anne Schuchat, MD, director of the CDC's National Center for Immunization and Respiratory Diseases, said at the news conference. "It came out with very reassuring results."

"Ninety-eight percent of the answers here are very reassuring," John Iskander, MD, MPH, acting director of the CDC's Immunization Safety Office, said at the news conference.

Thimerosal and Tics

What about the other 2% of the study findings? The study looked at 42 different outcomes, ranging from intelligence to fine motor coordination. The researchers performed 378 statistical tests on the data. Each statistical test had 5% odds of being a chance finding.

"By chance alone we would estimate about 5% of test results would be significant," Schuchat said. "And in fact that is exactly what we found: 19, or 5%, of the statistical tests did find significant results. They were pretty evenly split between [linking] better and worse outcomes [to thimerosal]."

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One of those worse outcomes was a slightly increased risk of tics -- repetitive physical motions or vocalizations. What makes the finding plausible is that two previous studies identified tics as a possible thimerosal side effect.

However, there is good reason to think that the tic finding was simply chance:

The significant finding for tics came from observers who saw the child only once. Parents of children with the highest thimerosal exposure reported no more child tics than parents of children with lower thimerosal exposure.

The finding applied to boys, but not to girls.

The tic finding included transient tics, which experts do not consider a problem.

WebMD asked pediatric neurologist Jonathan W. Mink, MD, PhD, to comment on the CDC study. Mink is chief of child neurology at the University of Rochester, N.Y., and co-chairman of the scientific advisory board for the Tourette Syndrome Association. He was not involved in the CDC study.

"Tics are seen quite commonly in kids with autism. But most kids with tics don't have autism," Mink tells WebMD. "The bottom line is that the rate of tics here is well within the rate we would expect to see in 7- to 10-year-old kids. Some of them have transient tics, which would go away, and some have tics that will never be a problem."

The bottom line, Mink says, is that the study gives "no reason to think that thimerosal or vaccination causes tics."

No Proof of Thimerosal Safety

The CDC asked more than a dozen outside consultants to comment on the study prior to publication of the final manuscript. One was Sallie Bernard, the parent of a child with autism and executive director of Safe Minds. Safe Minds is one of the groups working to convince parents that medical use of mercury causes autism and other child health problems.

Bernard formally dissents with the CDC's conclusion that the study "does not support a causal association between early exposure to mercury from thimerosal-containing vaccines and immune globulins and deficits in neuropsychological functioning at the age of 7 to 10 years."

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In a news release, Bernard points out that lack of evidence of thimerosal harm is not proof of thimerosal safety.

"The [CDC's] conclusion misleads the public, implying without qualification that a relationship has been disproved," Bernard says. "In fact, the study was unable to prove either the presence or absence of a causal relationship."

Bernard points to flaws in the study -- such as the inclusion of a relatively small number of children exposed to very high or very low amounts of thimerosal.